Method of incorporating amalgam-forming material in a fluorescent lamp

ABSTRACT

In the manufacture of a fluorescent lamp, a method of incorporating an amalgam-forming material, such as indium, as an integral part of the lamp for providing mercury vapor pressure regulation. During fabrication of the lamp mount structure, indium is applied to the hot flare of the glass stem immediately after the flare making step and prior to scoring and cutting. The non-flared end of the glass stem is then heated to the melting point and pressed to fuse a pair of lead wires and an exhaust tube into the stem. After completion of the mount, it is attached to the lamp by sealing the periphery of the stem flare to the end of the lamp envelope, and the lamp is further processed to completion.

United States Patent Latassa et al.

[451 Mar/l1, 1975 METHOD OF INCORPORATING AMALGAM-FORMING MATERIAL IN AFLUORESCENT LAMP [75] Inventors: Frank M. Latassa, Magnolia;

Howard W. Milke, Danvers; .Tadius T. Sadoski, Salem, all of Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

[22] Filed: Apr. 25, 1974 [21] Appl. No.: 463,952

[52] U.S. Cl. 29/25.l3, 316/25 [51] Int. Cl. HOlj 9/18, HOlj 17/26 [58]Field of Search 29/251, 25.11, 25.13,

[56] References Cited UNITED STATES PATENTS 3,048,737 8/1962 Rimbach313/174 3,287,587 11/1966 Menelly 313/178 X 3,548,241 12/1970' Rash eta1. 316/25 X Primary ExaminerRoy Lake Assistant Examiner.1. W. DavieAttorney, Agent, or FirmEdward .1. Coleman [57] ABSTRACT In themanufacture of a fluorescent lamp, a method of incorporating anamalgam-forming material, such as indium, as an integral part of thelamp for providing mercury vapor pressure regulation. During fabricationof the lamp mount structure, indium is applied to the hot flare of theglass stem immediately after the flare making step and prior to scoringand cutting. The nonflared end of the glass stem is then heated to themelting point and pressed to fuse a pair of lead wires and an exhausttube into the stem. After completion of the mount, it is attached to thelamp by sealing the periphery of the stem flare to the end of the lampenvelope, and the lamp is further processed to completion.

9 Claims, 7 Drawing Figures PATENTEU 1 75 HEAT SOURCE I 7 FiG.2

HEAT SOURCE METHOD OF INCORPORATING AMALGAM-FORMING MATERIAL IN AFLUORESCENT LAMP BACKGROUND OF THE INVENTION This invention relates tothe manufacture of lowpressure mercury vapor discharge lamps and, moreparticularly, to a method of incorporating an amalgamforming materialwithin the envelope of a fluorescent lamp at a location such that itregulates the mercury vapor pressure and permits the lamp to be operatedat high power loadings and under high ambient temperature conditionswith improved efficiency and lumen output.

It is well-known that the light output of a fluorescent lamp is afunction of the mercury vapor pressure, which in turn often depends uponthe temperature of the coldest region of the glass envelope of the lamp.It is further known that the envelope cold spot temperature for mostefficient lamp operation is approximately 40C, which causes a mercuryvapor pressure of approximately 4 to 6 10 Torr to occur inside the lamp.Often, due to high lamp loading or high ambient temperature, theenvelope temperature and mercury vapor pressure rise above the optimumvalue.

Various methods of cooling portions of the lamp envelope to regulatevapor pressure have beem em ployed. Shields have been placed between theelectrodes and the ends of the envelope; heat sinks have been attachedto the envelope; and the lamp envelope has been increased in size, andmade with grooves, depressions and the like. It has also been well-knownthat mercury vapor pressure may be reduced by the use of anamalgam-forming metal, such as cadmium or indium. U.S. Pat. No.2,966,602 mentions such an application of an amalgam of mercury atcolumn 4, lines 60-64. It has been observed that the location of theamalgam or amalgam-forming metal in the lamp is an important factor inproviding the desired improvement in lamp operation. For example, U.S.Pat. No. 3,007,071 discloses the use of an amalgam-forming metal as astrip or powder located along the length of the tubular lamp envelopewhere it is not exposed to temperatures much higher than those in thedischarge. In a lamp described by U.S. Pat. 3,392,298, the mercury vaporpressure is controlled by a coating ofindium in the form of a ring atthe center of the tubular lamp envelope. Such use of an amalgam-formingmetal is suitable for fixing the mercury vapor pressure after the lampreaches thermal equilibrium, but the vapor pressure in the lamp will beextremely low when the lamp is first started since a considerable timeis required for the middle part of the lamp, where the indium islocated, to warm up.

Accordingly, the use of two sources of amalgam within a fluorescent lamphas been employed-- one which heats up rather slowly when the lamp isenergized, and then controls the mercury vapor pressure duringoperation, and a secondary source of amalgam which is located closer tothe electrodes and thus heats up at a faster rate and provides asufficient amount of mercury vapor to enable the lamp to reach itsoutput more rapidly. A fluorescent lamp of this type is disclosed inU.S. Pat. No. 3,227,907. According to this patent, the cool spot depositof amalgam-forming material is provided by a band of indium at thecenter of the glass tube as described in U.S. Pat. 3,392,298, and

the hot spot deposit of indium is located on auxiliary electrodes, suchas flag anodes, connected to the cathode coil lead-in wires. Suchauxiliary electrodes generally rise quickly in temperature, and mayreach temperatures as high as 300C to 400C. Whatever mercury is pickedup by the indium in this location will be quickly vaporized into theatmosphere of the lamp and quickly diffused through it.

Although the above-discussed cool spot locations of amalgamsyviz alengthwise strip or deposit of amalgam-forming metal or a center band ofindium, can provide effective pressure regulation, it is difficult toapply the amalgam-forming metal with sufficient adherence at suchlocations on the interior surface of the lamp envelope, and since thelamp is heated to high temperatures during the preheating and bakingoperations, the amalgam has a tendency to melt and flow away from thedesired location. In addition, some amalgam-forming metals or amalgamshave such a low melting point that they become liquid at the operatingtemperatures within the lamp and thus create a situation where theamalgam may not remain at the desired location within the lamp. Further,the use of an amalgam center band or lengthwise strip will blockradiation and thus cause an appearance defect and some loss of lightoutput, which may be objectionable in specific applications. Also,amalgam locations toward the center of the lamp envelope are moresensitive to ambient temperature and thereby cause shifts in the mercurycontrol point of the lamp.

According to one approach for overcoming the abovementioneddisadvantages of prior cool spot locations of amalgam for providing themain vapor pressure control means within the lamp, a strip of theamalgamforming metal is placed in a wire mesh holder which is wrappedand tied or clamped about the cylindrical portion of the glass'mountstem at one or both ends of the lamp envelope. The temperature of theamalgam during lamp operation is dictated by the length of the stem andthe selected axial portion of the mesh wrap along the stem (and thus itsdistance from the electrode). Such a structure is described in thefollowing US. Pat. Nos: 3,373,303; 3,442,299; 3,526,802; 3,526,804;3,534,212 and 3,619,697. A disadvantage of this approach is that itcomplicates the lamp making process by the additional manual ormechanical steps required to properly place the indium into the wiremeshand attach the mesh about the mount.

In another approach to overcome the disadvantages of prior cool spotlocations, the main vapor pressure control means is provided bydepositing the amalgamforming material in a band about the glass mountstem at one or both ends of the fluorescent lamp. For example, U.S. Pat.No. 3,287,587 describes a method ofproviding such a band by rubbing apellet of indium against the barrel portion of a glass stem which hasbeen heated to a temperature of about 160C. A later reference, viz. U.S.Pat. No. 3,548,241, describes a method whereby a suitableamalgam-forming metal, such as indium, is heated to the liquid state andthen sprayed onto the flared portion of one of the glass mount stemsbefore it is sealed into the envelope. Preferably the stem is rotatedwhile the spray is controlled to-deposit a band of the amalgam-formingmetal having a thickness of less than microns which extends around thecircumference of the flare portion of the stem. Starting is facilitatedby an auxiliary (hot spot) source of amalgarn secured to an electrodecap (disintegration shield).

SUMMARY OF THE INVENTION In view of the foregoing, it is an object ofthe present invention to provide an improved method of manufacturinglow-pressure mercury vapor discharge lamps.

A principal object of the invention is to provide an improved method ofincorporating an amalgamforming material within a fluorescent lamp.

These and other objects, advantages and features are attained, inaccordance with the principles of this invention, by a methodcomprising: heating a vitreous tube from which the stem is to be made tothe softening point; flaring the heated stem tube at one end; andapplying the amalgam-forming material onto the exterior surface of theflared portion of the heated stem tube. Thereafter, an exhaust tube andlead wires are inserted into the stem tube, and the non-flared end ofthe stem tube is heated to the melting point and then is pressed to fusethe exhaust tube and lead wires therein. After the mount structure iscompleted, it is attached to the lamp by sealing the periphery of thestem flare to the end of the lamp envelope, and the remainingfabrication of the lamp is completed. Several advantages accrue fromthis method of incorporating the amalgamforming material. Little or nomodification of the automatic lamp and lamp mount processing equipmentare required. A minimum of material loss results if the flare isimproperly coated; i.e. only the flare, not a finished mount structure,must be discarded. The high temperature heating of the stem presssubsequent to application of an amalgam-forming material, such asindium, produces a more intimate indium to glass bond and serves todecontaminate the free indium metals from impurities and trapped gasesimmediately prior to lamp fabrication. A resulting oxide layer on theindium serves as a protective crust during lamp operation to reduce oreliminate the absorption of contaminants and prevent the softened indiumfrom running. Further, higher light outputs have been observed withlamps using stem flares precoated with indium in accordance with theinvention, as compared with lamps employing stem flares coated withindium after fabrication of the mount structure.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fullydescribed hereinafter in conjunction with the accompanying drawings inwhich:

FIG. 1 is a perspective view, partly in section, ofa fluorescent lampmade in accordance with the present invention, a portion of the bulbbeing removed for convenience;

and, in fabricating a mount structure according to the invention;

FIG. 2 illustrates a step of heating the end of the glass stem tube;

FIG.3 shows the stem tube after flaring one end;

FIG. 4 illustrates the application of indium to the hot stem flare bywire feed;

FIG. 5 illustrates the results of the score and cut operation;

FIG. 6 shows insertion of the exhaust tube and lead wires andillustrates heating of the non-flared end of the stem; and

FIG. 7 shows the partially fabricated mount structure after pressing andpiercing.

DESCRIPTION OF-THE PREFERRED EMBODIMENT Referring to FIG. 1, afluorescent lamp is shown comprising an elongated tubular glass envelope10 having the customary coating 11 of phosphor on its inner surface andelectrode mounts l2 sealed into each of its ends. The light-transmittingenvelope is filled with a small amount of rare gas, such as argon, atlow pressure, e.g., one to three Torr, and a small quantity of mercury,say 45 mgs., after which it is hermetically sealed in the usual mannerby tipping off the exhaust tube 13 (see FIGS. 6 and 7) at one or bothends of the lamp.

Each mount structure 12 includes the typical reentrant vitreous stem 14having at one end a flare portion 15 which is sealed about its peripheryto the end of the tubular glass envelope, and a press 16 at the inwardend supporting an electrode arrangement 17. More specifically theelectrode comprises a cathode coil 18, preferably consisting of a coiledtungsten filament carrying the usual alkaline-earth oxide(electron-emissive) coating, supported on a pair of lead wires 19 and 20sealed through the stem press 16 and extending to terminal pins 21 and22 insulatively mounted in the lamp bases 23 attached to each end of thehermetically sealed, light-transmitting envelope 10. A pair of flaganodes 24 and 25 are also mounted on either side of and in parallelrelationship with the cathode coil 18, preferably by attaching them tothe ends of the lead wires.

The lamp of FIG. 1 further includes a band 26 of amalgam-formingmaterial, such as indium, deposited on the flared portion 15 (cool spotlocation) of at least one of the mount stems 14 for regulating themercury vapor pressure when the lamp is in operation. Another coating 27of a much smaller quantity of amalgamforming material is located on atleast one of the electrically connected flag anodes (hot spot location)for improving the starting characteristics.

The quantities of amalgam-forming material required at each location arevariable from one lamp type to another. In addition, changes in thedesign of a lamp type (eg a change in the heat output ofa coil, or achange in the mercury content) may necessitate changes in indiumquantities or placement. Satisfactory results have been obtained inF40Tl2 lamps employing approximately 2.5 mgs. of indium on each of fourflags in a lamp (it may range from 3 to 10 mgs. of indium per inch offlag length) and from 40 mgs. to I40 mgs. of indium on each of twoflares per lamp. The mercury content of the lamps was approximately 45mgs. As previously mentioned, the main location of indium need not beapplied to both flares. Also, the hot spot location of indium forfacilitating starting may be applied to one flag rather than all fourflags.

In accordance with the present invention, the main deposit of amalgamforming material, viz., band 26, is incorporated in the lamp of FIG. 1by the following method of manufacture to provide a number of significant advantages with respect to reduced manufacturing cost, a morereliable lamp construction, and improved operating results. Withreference to the fabrication of a single electrode mount structure 12, atube of glass or some other suitable vitreous material (see FIG. 2),from which the stem 14 is to be made, is initially heated at one end tothe softening point. Next, the heated end of the glass stem tube isflared to provide portion (see FIG. 3). The next step on a conventionalflaremaking machine would be a cutting and scoring operation to removeexcess tubing, after which the mount 12 would be completed and theindium band would be applied prior to joining the mount to the lampenvelope. According to the present invention, however, the indium, orother amalgam-forming material, is applied to a hot flare on theflare-making machine after the flaring operation and before the scoringoperation. Typically, the heated stem is rotated about its longitudinalaxis on the flare-making machine, and the amalgam-forming material isapplied inwardly from the rim of the flare 15 so that it is depositednear the bend region of the rotating flare in a layer of annularconfiguration, viz. band 26, that extends around the circumference ofthe flare. For example, as illustrated in FIG. 4, a preferred method ofapplying the indium, or other amalgamforming material, is to feed it asa wire 28 onto the hot flare portion 15 of the rotating stem tube. Therate of feed can control the specific amount of indium deposited. Withsmaller quantities of indium, it might be more desirable to cover only asegment of the flare circumference.

An alternative method of application is to use indium powder in 5suspension and spray it onto the hot rotating flare. The weight of thedeposited indium can then be controlled by the size of the nozzle andspraying time. Spraying of liquid indium metal could also besuccessfully used since the indium is being applied to an unfinishedmount flare which allows for improved indium to glass bonding throughsubsequent high temperature heating of the flare in mount fabrication.Again, other amalgam-forming materials may be used in lieu of indium inthe above described spray methods of application.

After applying the amalgam-forming material to the flare, the rotatingstem tube is scored and cut to re move a segment 29 of the nonflared endthereof (see FIG. 5). Of course, the cutting step may be unnecessary ifthe desired stem length is used in the flare forming process. Next, anexhaust tube 13 and the pair of lead wires 19 and are inserted into theflared and cut stem tube, and the non-flared end of the stem tube isheated to the melting point (see FIG. 6) in open air. Then, the very hotnon-flared end of the stem tube is pressed to fuse the exhaust tube 13and lead wires therein. After piercing a hole through the hot press 16into the exhaust tube 13, the partially fabricated mount appears asshown in FIG. 7. When the mount is heated to these high temperatures inthe mount fabrication process in open air, a quite noticeable, darkoxide layer forms on the otherwise silvery appearing indium band. Thishas proved to be considerably advantageous in that it provides aprotective crust over the indium, as will be discussed in more detailhereinafter.

Thereafter, fabrication of the mount structure is completed by shapingthe leads extending from the press, attaching the cathode coil 18 acrossthe ends of the lead wires 19 and 20, such as by clamping, and thenattaching the flag anodes 24 and 25 to the lead wires, such as bywelding, with the coating 27 of indium being applied to the outsidesurface of each flag anode either before or after attachment. Forvarious lamp types and applications, of course, the flag coating 27 maynot be required, or flag anodes may not be employed.

The described method of applying the primary deposit of amalgam-formingmaterial (band 26) after flaring and before scoring provides severalsignificant advantages. The only additional step in the normal lampmaking process is the insertion of a wire-feed or nozzlespray operationwhile the stem tube is still on the flare making machine and, thus,early in the fabrication of the mount structure. Otherwise, there is nomodification of automatic lamp and lamp mount processing equipmentrequired, and the indium application (band 26) requires no other changein the normal sequence of manufacturing the lamp or lamp parts. Further,there is a minimum of material loss if the flare is im properly coated,as only the flare and not the finished mount need be discarded.

Application of the indium band to the hot flare before scoring andcutting allows for subsequent high temperature heating of the flare inthe normal mount process (the non flared end of the stem is heated tothe melting point prior to pressing) which produces a more intimateindium to glass bond. Further the normal heating of the flare duringmount fabrication, particularly the heating of the non-flared end of thestem to the melting point prior to pressing, serves to outgas and de'contaminate the indium band 26 from impurities and trapped gasesimmediately prior to lamp fabrication. Further, the distinctive oxidelayer over the indium band 26, resulting from the high temperatureheating and pressing operations, provides a protective crust on the band26 as the indium oxide has a higher melting point than the pure metal.Accordingly, when the indium is softened during sealing of the mountflare to the end of the glass envelope, the harder oxide overcrust willsignificantly reduce or eliminate the contaminants picked up by the softindium. In addition the oxide layer helps to prevent the indium fromrunning during lamp processing and operation. Of course. pre viousconventional methods of applying the oxide band 26, such as sprayingonthe completed mount structure prior to sealing the lamp envelope, doleave a trace of oxide layer which is barely visible, however, the abovenoted advantages, obtainable with the obviously much heavier oxide layerproduced by the present method invention, are not evident. The minimalnature of the oxide layer produced in conventional methods during thelamp sealing process is due to the fact that the indium is shielded bythe glass envelope, tangential fires in the sealing operation heat avery small portion of the mount flare at its end producing less heattoward the indium layer, and the glass envelope acts as a heat sink.

Finally, testing oflamps made in accordance with the invention, ascompared to the same type lamps with the indium applied to the sternflare after mount fabrication but before sealing the mount to theenvelope, has exhibited a definite 2 to 3 percent increase in lightoutput over an ambient temperature range of from 45F to F.

Although the invention has been described with respect to specificembodiments, it will be appreciated that modificationsand changes may bemade by those skilled in the art without departing from the true spiritand scope of the invention.

What we claim is:

1. in the manufacture of a low-pressure mercury vapor discharge lamphaving an electrode mount structure including a vitreous stem sealed toand extending inwardly from one end of the lamp envelope, the

method of incorporating an amalgam-forming material as an integral partof the lamp and at a location therein such that said material willregulate the mercury vapor pressure when the lamp is operated, saidmethod comprising:

heating a vitreous tube from which said stem is to be made to thesoftening point thereof,

flaring said heated stem tube at one end,

applying said amalgam-forming material "onto the exterior surface of theflared portion of said heated stern tube,

inserting an exhaust tube and lead wires into said stem tube,

heating the non-flared end of said stem tube to the melting pointthereof, pressing the non-flared end of said heated stem tube to fusesaid exhaust tube and lead wires therein,

completing fabrication of the electrode mount structure which includessaid stem tube, exhaust tube and lead wires, and

attaching said mount structure to the lamp by sealing the periphery ofthe flared portion of said stem tube to an end of said lamp envelope,and then completing the fabrication of the lamp.

2. The method of claim 1 wherein said amalgamforming material is appliedinwardly from the rim of said flared portion while said heated stem tubeis being rotated about its longitudinal axis so that siad material saiddeposited in a layer of annular configuration that extends around thecircumference of said flared portion.

3. The method of claim 2 including the further steps of scoring andcutting said rotating stem tube to remove a segment of the non-flaredend thereof subsequent to applying the amalgam-forming material andprior to'inserting the lead wires and exhaust tube.

4. The method of claim 2 wherein said amalgamforming material is appliedby feeding a wire thereof onto the heated flared portion of saidrotating stem tube.

5. The method of claim 4 wherein the amalgamforming material is indium.

6. The method of claim 1 including the further steps of scoring andcutting said stem tube to remove 21 segment of the non-flared endthereof subsequent to applying the amalgam-forming material and prior toinserting the lead wires and exhaust tube.

7. The method of claim 1 wherein said vitreous tube is glass, one ofsaid glass tube is heated to the softening point thereof prior to saidflaring step, and said amalgam-forming material is indium and is appliedinwardly from the rim of said flared portion about the circumferencethereof in a layer of annular configuration.

8. The method of claim 1 wherein said amalgamforming material is appliedby using a powder thereof in a suspension and spraying it onto theheated flare portion of said stem tube.

9. The method of claim 1 wherein said amalgamforming material is appliedby spraying a liquid thereof onto the heated flare portion of said stemtube.

1. IN THE MANUFACTURE OF A LOW-PRESSURE MERCURY VAPOR DISCHARGE LAMPHAVING AN ELECTRODE MOUNT STRUCTURE INCLUDING A VITREOUS STEM SEALED TOAND EXTENDING INWARDLY FROM ONE END OF THE LAMP ENVELOPE, THE MTHOD OFINCORPORATING AN AMALGAM-FORMING MATERIAL AS AN INTEGRAL PART OF THELAMP AND AT A LOCATION THEREIN SUCH THAT SAID MATERIAL WILL REGULATE THEMERCURY VAPOR PRESSURE WHEN THE LAMP IS OPERATED, SAID METHODCOMPRISING: HEATING A VITREOUS TUBE FROM WHICH SAID STEM IS TO BE MADETO THE SOFTENING POINT THEREOF, FLARING SAID HEATED STEM TUBE AT ONEEND, APPLYING SAID AMALGAM-FORMING MATERIAL ONTO THE EXTERIOR SURFACE OFTHE FLARED PORTION OF SAID HEATED STEM TUBE, INSERTING AN EXHAUST TUBEAND LEAD WIRES INTO SAID STEM TUBE, HEATING THE NON-FLARED END OF SAIDSTEM TUBE TO THE MELTING POINT THEREOF, PRESSING THE NON-FLARED END OFSAID HEATED STEM TUBE TO FUSE SAID EXHAUST TUBE AND LEAD WIRES THEREIN,COMPLETING FABRICATION OF THE ELECTRODE MOUNT STRUCTURE WHICH INCLUDESSAID STEM TUBE, EXHAUST TUBE AND LEAD WIRES, AND ATTACHING SAID MOUNTSTRUCTURE TO THE LAMP BY SEALING THE PERIPHERY OF THE FLARED PORTION OFSAID STEM TUBE TO AN END OF SAID LAMP ENVELOPE, AND THEN COMPLETING THEFABRICATION OF THE LAMP.
 1. In the manufacture of a low-pressure mercuryvapor discharge lamp having an electrode mount structure including avitreous stem sealed to and extending inwardly from one end of the lampenvelope, the method of incorporating an amalgam-forming material as anintegral part of the lamp and at a location therein such that saidmaterial will regulate the mercury vapor pressure when the lamp isoperated, said method comprising: heating a vitreous tube from whichsaid stem is to be made to the softening point thereof, flaring saidheated stem tube at one end, applying said amalgam-forming material ontothe exterior surface of the flared portion of said heated stem tube,inserting an exhaust tube and lead wires into said stem tube, heatingthe non-flared end of said stem tube to the melting point thereof,pressing the non-flared end of said heated stem tube to fuse saidexhaust tube and lead wires therein, completing fabrication of theelectrode mount structure which includes said stem tube, exhaust tubeand lead wires, and attaching said mount structure to the lamp bysealing the periphery of the flared portion of said stem tube to an endof said lamp envelope, and then completing the fabrication of the lamp.2. The method of claim 1 wherein said amalgam-forming material isapplied inwardly from the rim of said flared portion while said heatedstem tube is being rotated about its longitudinal axis so that saidmaterial is deposited in a layer of annular configuration that extendsaround the circumference of said flared portion.
 3. The method of claim2 including the further steps of scoring and cutting said rotating stemtube to remove a segment of the non-flared end thereof subsequent toapplying the amalgam-forming material and prior to inserting the leadwires and exhaust tube.
 4. The method of claim 2 wherein saidamalgam-forming material is applied by feeding a wire thereof onto theheated flared portion of said rotating stem tube.
 5. The method of claim4 wherein the amalgam-forming material is indium.
 6. The method of claim1 including the further steps of scoring and cutting said stem tube toremove a segment of the non-flared end thereof subsequent to applyingthe amalgam-forming material and prior to inserting the lead wires andexhaust tube.
 7. The method of claim 1 wherein said vitreous tube isglass, one of said glass tube is heated to the softening point thereofprior to said flaring stEp, and said amalgam-forming material is indiumand is applied inwardly from the rim of said flared portion about thecircumference thereof in a layer of annular configuration.
 8. The methodof claim 1 wherein said amalgam-forming material is applied by using apowder thereof in a suspension and spraying it onto the heated flareportion of said stem tube.